1. Field of the Invention
The present invention relates to a pocket-forming step and a related preparative step in PTP (Press-through-packaging) machines. In particular, it is concerned with a method and an apparatus for forming pockets on a newly-developed unrolled PTP sheet which has high moisture-resistant but requires delicate care in handling, particularly in molding operations performed in order to form pockets which accommodate articles to be wrapped individually.
2. Description of the Prior Art
In the early promoting stage, polyvinyl chlorides (PVC) were mainly used as unrolled sheets for PTP and are still occupying the predominant position even at present. The predominance can be attributable to the very easy molding property of PVC resulting from an extremely broad range of molding temperatures i.e., 82.degree.-212.degree. C. and the permitted coarse temperature adjustment of heating means, say, .+-.10.degree. C. in generally-accepted compressed-air blowing methods. And a simple heating means such as passage through a radiation tunnel may be used in the preparative heating without any trouble.
In the case of a monolayer PVC sheet, widely adopted compressed-air blowing may be employed for forming pockets in a manner wherein the evenly heated unrolled sheet is extended over a metal female die and compressed-air is blown to pocket regions of the sheet to inflate the specified regions until the outer surface of the regions comes in smooth and intimate contact with the inner surface of the female die.
Such an excellent molding property of PVC may also be attributable to its amorphous property and the method of the stated level, which can dispense with the use of any solid male die or plug, can be employed without any trouble.
However, the moisture-resistant property of PVC is insufficient when the level of the required property is set very high in such case wherein the articles to be packaged are, for instance, medicines. An additional gas-tight wrapping is required but it does not however guarantee the quality of the articles once the wrapping has been loosened.
On account of this difficulty, attempts have heretofore been made, one after another, for adopting a monolayer sheet of polyethylene (PE) or polypropylene (PP), or various laminated sheets. The moldings of these sheets are however by no means easy because the temperature ranges permitted for the molding operation narrows gradually as the material varies, for instance; from 125.degree. C. to 155.degree. C. for a PP monolayer sheet and from 124.degree. C. to 135.degree. C. for a multiple laminated sheet having at least one core layer of high density PE.
When the object to be molded is PP, which is a crystalline high molecular substance, it is difficult to follow such a method because PP has a narrower temperature range for molding and poor molding properties. That is, evenly heating the sheet may cause troubles during transfering because the even heating sometimes means excessive softening which may invite "draw-down" of the sheet along the transfering path. On the other hand, if the sheet as a whole is heated only to an extent of effectively avoiding the transferring troubles, it may cause insufficient softening which sometimes results in an unacceptable uneven thickness of the deeply drawn pockets.
Namely, a conventional preparative heating by passing the sheet through a radiation tunnel is insufficient for performing stable transferring and high speed molding of such PP sheet, not to mention difficulty in temperature controlling.
As a result of the previously described reasonings, the method has gradually been replaced by a method wherein the unrolled sheet is repeatedly sandwiched between a pair of reciprocating solid plates to be conductively heated. The heat transfer by conduction has an auxiliary advantage in that it is easy to heat only pocket-forming regions locally and discriminately to leave the rest intact as a lattice. Therefore, the sheet after being heated has a "lattice" structure which can withstand the tension developed in the direction of the sheet's travel.
The standard mode of preparative heating by conduction is sandwiching the sheet supplied by intermittent transfering between a pair of heating plates and repeating the heating until the core temperature of the sheet reaches a saturation temperature, i.e. that equals the temperature of the heating plates. It is particularly important in handling materials of narrow temperature range, wherein molding is permitted, such as PE monolayer sheet and a multiple laminated sheet having at least one core layer of high density PE. Therefore, the length of heating plates and number of repetitions (tact), when that of the molding step is set to a standard (one tact), inevitably become greater and the requirement for the uniform temperature of the plates becomes correspondingly stricter.
Attempts have been made for the required uniform heating and for equalizing temperature of the large heating plate by adjusting the distribution of electric power consumed is embedded heaters or by adopting a means of embedding a heat pipe or pipes in place of the electric heaters. An abnormal localized rise in temperature at, for instance, places remote from the supporting means which hold the heating plates on a vertical mainframe of the machine could not still however be avoided and it was difficult to satisfy the functional requirement of fine temperature adjustment, say .+-.1.5.degree. C., over the entire heating plates.